This picture shows how to remove and reinstall the tensioner. The E10 is to pull the tensioner back to install or remove the belt. The arrow is pointing where you need to be. There is a round black plastic cover that you pop off with a screwdriver. It will be tight. Behind that cover is the nut. I forgot to mention the cover previously. Sorry about that. You should be good to go now.
Mercedes Benz Master Tech
ASE Master Technician
Owner of "Behind the Star Inc."